The invention relates to a method and an arrangement for operating a steering system for a motor vehicle.
Such a steering system is shown in DE-OS 40 31 316 (corresponds to U.S. Pat. No. 5,205,371) and will be explained with respect to FIGS. 1 and 2 insofar as relevant for an understanding of the present invention. In a steering system of this kind, the steering movements, which are developed by the driver via the steering wheel 11 or 21 (that is, the steering wheel angle xcex4L, which is detected by the sensor 28), are superposed in the superposing gear assembly 12 or 22 with the movements of the actuator 13 or 23 (that is, the motor angle xcex4M) The superposed movement xcex4Lxe2x80x2, which so arises, is transmitted via the steering transmission 14 (or the steering rod 16) to the wheels 15a and 15b, which are configured so as to be steerable, to adjust the steering angle xcex4V. Here, the actuator (13 or 23) is configured as an electric motor. The principal of operation of such a servo steering system comprises that the steering can be made very indirect via the transmission ratio iu of the superposing gear assembly 12 or 22 and thereby attain small steering wheel torques ML. Very large steering wheel angles xcex4L, which are caused in this way, are avoided in that suitable motor angles xcex4M are superposed so that, in correspondence to the relationship
xcex4Lxe2x80x2=xcex4L/iu+xcex4M,
output angles xcex4xe2x80x2 can be adjusted required with conventional size steering wheel angles. The motor angle xcex4M, which is required to support steering, or its desired value, is determined from the steering wheel angle xcex4L. Furthermore, the motor angle xcex4M can also be selected in dependence upon signals Sm. These signals Sm represent the vehicle movements detected by the sensors 26.
Furthermore, it is generally known that the steering wheel angles xcex4V are limited by so-called steering stops depending upon construction.
The object of the present invention is to consider the steering stops for such a superposing steering with respect to increased safety and comfort.
As mentioned, the invention relates to a steering system for a motor vehicle having at least one steerable wheel, an actuator and a superposing gear assembly. By means of the superposing gear assembly, the steering movement initiated by the driver of the vehicle and the movement, which is initiated by the actuator, for generating the steering movement of the steerable wheel is superposed within pregiven steering stops.
The essence of the invention is to reduce the movement, which is initiated by the actuator, before reaching a steering stop.
According to the invention, no further additional angle can be superposed when reaching the stops delimiting the steering wheel angles. Otherwise, the steering wheel is rotated against the steering wheel rotational direction which was present previously whereby the driver is greatly irritated. With the superposition of the additional angle up to close to the stops, the above-mentioned problem of the steering wheel return rotation, however, occurs which is effectively prevented by the run-out of the invention of the steering wheel support in the region of the steering stops. This superposition of the additional angle takes place especially with rapid steering movements such as when turning in three strokes because of the after-running of the actuator caused by inertia (for example, the after-running of the motor armature when using an electric motor). With the invention and with a power steering pursuant to the movement superposition principal, one obtains added safety because the driver is not irritated by an unexpected rotation of the steering wheel. Furthermore, the application of the invention means added comfort because of the continuous run-out of the steering support.
In an advantageous configuration of the invention, the movement, which is initiated by the actuator, is reduced to the value zero before reaching a steering stop (that is, up to the standstill of the position drive). In the region of the steering stops therefore no servo support takes place.
Furthermore, it can be provided that the movement, which is initiated by the actuator, is reduced before reaching a steering stop by means of a pregivable or pregiven characteristic, especially to the value zero.
It is especially advantageous to reduce the movement, which is initiated by the actuator, within a predetermined region before reaching a steering stop and especially to reduce this movement to the value zero. This reduction can also take place with a pregiven or pregivable characteristic.
Furthermore, it is very advantageous that, after the reduction of the movement (which is initiated by the actuator), the actuator is switched off before reaching a steering stop and/or is braked. For this purpose, it can be especially provided that the switchoff and/or braking is done in dependence upon a comparison of the detected steering movement of the steerable wheel and/or of the detected steering movement, which is initiated by the driver of the vehicle, to a pregivable or pregiven threshold value. In this configuration, a two-stage procedure is suggested in the region of the steering stops. First, in a range forward of reaching a steering stop, a reduction of the actuator movements takes place and, thereafter, a switchoff and/or braking is actuated. This has the background that, for the control of the actuator, control deviations and/or a certain after-run can occur so that a simple driving of the actuator in the sense of a reduction is not sufficient in order to bring it to standstill when reaching a steering stop.
Reaching the steering stops and/or the start and/or the end of the reduction of the movement, which is initiated by the actuator, can take place with a detection of the steering movement of the steerable wheel and/or via a detection of the steering movement which is initiated by the driver of the vehicle.
Furthermore, it is especially provided that the above-mentioned range is determined by at least one threshold value for the steering movement of the steerable wheel and/or for the steering movement initiated by the driver of the vehicle.